What happened on the day the dinosaurs went extinct? Sturgeons know the answer

Sixty-six million years ago, an ornithischian plant-eating dinosaur named Thescelosaurus was strolling along the coast of what is now North Dakota.

Skeleton model of Strange Dragon | Arv nm / wikimedia

Suddenly it froze, and looked around nervously - the ground began to shake, the sea surged, and a 9-meter-high wave was gradually forming from the south, heading north, swallowing everything. The strange dragon began to flee in panic, but it was all in vain. It and many other animals were instantly swept into the destructive sea waves, mud and debris mixed with floods , and the huge waves ruthlessly tore its legs off its body.

This strange dragon will never know that in the next 24 hours, the dinosaur family represented by it will face a little friction caused by an alien visitor - this alien visitor is the huge Chicxulub asteroid , which is believed to be the culprit of the extinction of dinosaurs.

An artist's impression of a meteorite impact event | Donald E. Davis

It would have no idea that tens of millions of years later, one of its scorched legs would become important evidence for a creature called "human" to understand the impact.

Strange Dragon: Poor Grilled Dragon Leg

In 2021, Robert DePalma, a paleontologist at the University of Kansas, led a team to conduct a scientific expedition. At a fossil site called Tanis site in the Hell Creek Formation in North Dakota, they discovered a dinosaur leg . Although the surface of the leg was carbonized, the skin, muscles and bones were exceptionally well preserved .

The unfortunate mummy fossil of the strange dragon | Documentary "Dinosaurs: The Last Day"

In order to identify the fossil, DePalma took it to London and sought the opinion of Paul Barrett, an expert on ornithischian dinosaurs. Barrett believed that it was the leg of a strange dragon , the victim mentioned at the beginning of the article. It is not particularly rare to find a mummified dinosaur. What shocked DePalma was the truth revealed by the details of this dinosaur leg - it was likely a witness and victim of the asteroid impact .

Map of Tanis Point | Robert A Depalma et al. / Proceedings of the National Academy of Sciences

Because the fossil is highly complete, DePalma and Barrett were able to try to infer what happened to the dinosaur. Barrett noticed that the pelvis and tail vertebrae of the dinosaur overlapped partially, and they suspected that the animal might have been violently rolled and buried quickly like the victims of Pompeii . The most likely possibility is that the flood caused by the asteroid impact rolled and tore the poor animal, eventually forming the fossils we see today.

But why did DePalma and Barrett conclude that the flood encountered by this unfortunate dinosaur was caused by an asteroid hitting the earth? This has to start with the origin of Tanis.

Not always a lucky survivor

The Tanis fossil site was first discovered by a team led by Steve Nicklas, a professor at the University of North Georgia. DePalma began excavating and studying the site in 2012. In 2017, DePalma published a striking research result here: the Tanis fossil site is a witness to the extinction of dinosaurs .

Why did mammals survive the extinction of dinosaurs and become the masters of the new era? The mainstream theories in the past included that mammals are small in size, highly adaptable, not picky eaters, have a constant body temperature, and dig holes. Among them, the burrowing theory is a more popular theory, because burrowing is quite common among modern small mammals and is an effective survival strategy. However, this point has always lacked fossil evidence to support it.

Fossils found at Tanis Fossil Site | NASA Goddard Space Flight Center

De Palma’s team announced that they had found a fossil burrow at Tanis that would help support that theory: a long, narrow, interconnected flask-shaped chamber, dotted with fossils of small mammals called Cimolesta (“chalk thieves”) that would undoubtedly have been residents of the cave system .

The cave is filled with sediments, which contain spores and pollen of many local ancient plants (such as pine and fern), which shows that when the sediments were poured into the cave, there were still a lot of plants outside. After the asteroid impact, the plants were destroyed, so the spores and pollen outside the cave are very few.

An image of the extinction of the dinosaurs. The world outside the cave has become a scorched earth. | National Science Foundation, Zina Deretsky

Based on the plant sediments, people can guess the origin of this cave - animals quickly dug this cave in a very short time before and after the asteroid impact, and then abandoned it . The filling is like a time capsule, preserving some materials before the catastrophe. The owner of this cave did not survive through the shelter of the cave like other mammals. Unfortunately, it and its shelter were heavily blocked by the dust brought by the meteorite impact, and like many animals, it was annihilated in the historical torrent of the late Cretaceous period.

The significance of the discovery of this cave, in addition to proving that ancient mammals were indeed able to survive the apocalyptic catastrophe by digging holes, also suggests that the animals in the Tanis area may have been direct witnesses of the Chicxulub meteorite impact !

Fish: Cradle of Death

At the end of the Late Cretaceous, the Tanis region was full of rivers and swamps, a paradise for aquatic animals. The sturgeon fossils here will further reveal what happened in the world before and after the asteroid impact.

DePalma's colleague Melanie During visited the Tanis fossil site several times in 2017. She was very confident that she had found a fish "eyewitness" on the day of the impact - a sturgeon of six tails on the day of the impact.

Acipenser truncatus, this is certainly not the fish in question, but the history of the sturgeon family is indeed very long｜Cliff / Wikimedia

The data results of fossil tomography show that the gill rakers of these sturgeons preserve a large number of extraterrestrial dust particles , and these extraterrestrial particles have only one source - the dust particles scattered after the impact of the Chicxulub meteorite.

These particles have been filtered out by the sturgeons before they entered their mouths or further into their digestive tracts, which means that they died before they had time to put these things into their mouths . Their death was probably caused by the violent lake quake caused by the meteorite impact, which caused the river to suddenly surge and thus caused the sudden death of these fish.

A fossilized sturgeon skull. The yellow dots in the green part are dust trapped in the gill cavity | Melanie Aranka Dominique During et al. / Nature (2022)

After using micro-X-ray technology to analyze the preservation of the specimens, Du Rui's team found that these sturgeons only experienced a very short period of diagenesis to form fossils, which is very likely the result of being quickly buried after the meteorite impact . If these sturgeons are indeed "eyewitnesses" of the impact date, paleontologists and geologists can follow the clues and use the age of the layers where these fossils were unearthed to make a more detailed and specific judgment on when the meteorite that caused the extinction of dinosaurs 66 million years ago hit the earth, and even in what season.

Extinction occurred in one spring

66 million years ago, the huge Chicxulub meteorite hit the Yucatan Peninsula in Mexico today. Its impact directly affected everything within a radius of 1,500-4,000 kilometers from the impact point, and the Tanis region, located 3,000 kilometers away from the impact point, was completely covered by it.

The chain reaction caused by the meteorite impact led to the complete extinction of 75% of the species on Earth, marking the end of the Mesozoic Era. In the past, people called this extinction event the Cretaceous-Tertiary extinction event , or KT extinction event for short; they also regarded 65 million years ago as the end of the Cretaceous period.

Mineral layers deposited during the KT extinction event (white lines) | Zimbres / wikimedia

But in 2004, due to the lack of exact equivalence of the time span covered by the Tertiary, the International Commission on Stratigraphy no longer recognized the validity of the Tertiary and split the Tertiary into the Paleogene and Neogene. Naturally, the International Commission on Stratigraphy encouraged the use of the Cretaceous -Paleogene extinction event ( K-Pg extinction event ) to replace the name "KT extinction event". The boundary between the Cretaceous and Paleogene was also determined to be around 66 million years ago, but the public has not yet adapted to this new geological concept.

Back to Tanis, in order to figure out in which season the K-Pg event occurred, Dury and her colleagues measured the sturgeon fossils of Tanis. They found that the main growth seasons of these fish are roughly spring (March-June) and autumn (September-December), because the number of zooplankton, the food of sturgeons, reaches a peak at this time. Dury's team determined the growth of these sturgeons by analyzing the carbon isotope content, so that they could know the season when they were buried in the meteorite impact .

Thin section of a fossil sturgeon tooth | Melanie Aranka Dominique During et al. / Nature (2022)

The analysis results revealed that 66 million years ago, the Chicxulub meteorite hit the Gulf of Mexico in North America in the spring of the northern hemisphere . That is to say, the dinosaurs may have gone into the abyss of extinction at the turn of spring and summer!

Spring should be a season of revival and vitality. However, in this spring, almost all animals weighing more than 50 kilograms died in the catastrophe brought by the Chicxulub meteorite . This is because most animals in the northern hemisphere have just recovered from the resource-scarce winter, and they can no longer survive the second severe winter brought by this alien visitor.

Fortunately, 66 million years after the impact, we finally found the first eyewitnesses of the day - although they were a few fish and the remaining legs of a dinosaur. I believe that in the near future, in addition to the Tanis area, more in-depth fossil evidence will be discovered around the world, little by little, completing the puzzle left to us by the meteorite 66 million years ago.

References

[1] During, MAD, Smit, J., Voeten, DFAE et al. (2022). The Mesozoic terminated in boreal spring. https://doi.org/10.1038/s41586-022-04446-1

[2] DePalma RA, Oleinik AA, Gurche LP, Burnham DA, Klingler JJ, McKinney CJ, Cichocki FP, Larson PL, Egerton VM, Wogelius RA, Edwards NP, Bergmann U, Manning PL. (2021). Seasonal calibration of the end-cretaceous Chicxulub impact event. https://doi.org/10.1038/s41598-021-03232-9

[3] DePalma, Robert A.; Smit, Jan; Burnham, David A.; Kuiper, Klaudia; Manning, Phillip L.; Oleinik, Anton; Larson, Peter; Maurrasse, Florentin J.; Vellekoop, Johan; Richards, Mark A.; Gurche, Loren; Alvarez, Walter (2019). A seismically induced onshore surge deposit at the KPg boundary, North Dakota. https://doi.org/10.1073/pnas.181740711

[4] DePalma, R. et al. (2017) Life after impact: A remarkable mammal burrow from the Chicxulub aftermath in the Hell Creek Formation, North Dakota

[5] The Day the Dinosaurs Died,Douglas Preston,March 29, 2019

[6] Marshall, Michael (1 April 2019). "Incredible fossil find may be first victims of dino-killer asteroid"

[7] "Tanis: Fossil of dinosaur killed in asteroid strike found, scientists claim". BBC News. 6 April 2022.

[8] Amos, Jonathan (24 February 2022). "'Dinosaur asteroid' wrought springtime devastation"

[9] Schulte, Peter (5 March 2010). "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary" DOI: 10.1126/science.1177265

Author: Insomnus

Edit: Red Queen

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